It's a safe wager that professional poker players aren't very good writers, but it's also better than even money that adept writers are, or could be, cunning poker players, for they come to understand motive and risk and instinctively realize that you can't win if you don't bet. James McManus bet big and won. His ''Positively Fifth Street,'' an exhilarating chronicle of the 2000 World Series of Poker in Las Vegas, will go on the shelf with the classic that inspired it, ''The Biggest Game in Town,'' A. Alvarez's account of the 1981 event.

Harper's Magazine sent McManus, a novelist who teaches at the School of the Art Institute of Chicago, to report on the poker tournament. He received an advance for expenses but had another idea. ''My ulterior plan was to use the $4,000 check,'' he writes, ''to try to play my way into the championship,'' which costs $10,000 to enter.

McManus began playing poker when he was 9 and participates in a local game at which $1,000 pots are common. Obviously he loves the game, and he's not alone. Some 60 million to 80 million Americans play. Why? Poker, like that other great American pastime with a World Series, is something you either get or you don't. And poker is a lot like baseball. Just listen to McManus: ''In both contests position, aggression and stealing are critical, but patience is what turns out to be the most necessary virtue. Baseball and poker players spend most of their time waiting, getting into position and strategizing, but once every nine chances or so . . . they really do have to come through.'' If you don't see that analogy, McManus is capable of raising you, going highbrow, quoting, for example, the scholar John Lukacs, who called poker ''the game closest to the Western conception of life . . . where men are considered moral agents, and where -- at least in the short run -- the important thing is not what happens but what people think happens.''

Betting large amounts of money can induce guilt, of course, and to deal with that issue McManus divides his personality into Good Jim and Bad Jim. In offering up reasons, or at least humorous rationalizations, for his attraction to gambling, Bad Jim, never immune to digressions, is not above invoking such things as the genius of Dostoyevsky, game theory, even evolutionary biology: ''Macho one-upmanship strikes many people as trivial and pathetic, and no doubt they're right, but it's also a fact that without this competitive ardor, humans would be long extinct. One reason we're not is that the sociobiological legacy of our days as hunter-gatherers causes levels of testosterone, the primary hormone of both male and female desire, to rise by as much as a third when the local team does well. He shoots, he scores! And later that night, so do we.'' But Bad Jim also has some excuses for gambling that make sense. Writing poetry, he says, is ''a habit that's much more expensive. The most I have earned for a poem is $100, and that poem took eight months to write. Usually I earn much, much less. Yet I'm treated to no sermons about that risky, acumen-proving habit.'' Who can fault that logic?

Good Jim is conscious of, and sensitive to, his large mortgage, how deep he is into his home equity line, his children and his extremely understanding wife. ''A responsible journalist,'' he tells her when he reveals his plan not only to cover the championship but also to play in it, ''needed actual table experience to capture the rhythms and texture of the hair-raising brand of No-Limit Texas Hold 'Em that decides the world championship, right? Krakauer on Everest, I mentioned. McPhee in Alaska.'' You can picture her expression. Still, Good Jim never bets the grocery money. He pays for his gambling with his winnings and with the money he makes writing essays and book reviews. It's a fund to which this publication has occasionally contributed, albeit rather minutely.

To prepare himself for the competition, McManus bones up on the game (if you're ever crazy enough to want to play poker with the pros, he tells you which books are essential reading) and plays tens of thousands of hands against computer programs. These poker programs will beat the average player. But unlike their brethren, the chess programs, they can't compete with the best players. That's because, unlike chess, poker involves deception, cheating really. Despite all their advances, computers still have a hard time distinguishing between erratic play (stupidly misplaying a hand) and stealing a huge pot with a spectacular bluff.

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Then the two Jims are off to Las Vegas and Binion's Horseshoe Casino. Benny Binion began gambling operations in Texas until for complicated reasons he left (''My sheriff got beat in the election'' is how he summed it up). In 1946 he piled his family and $2 million in cash into his Cadillac and drove to Las Vegas, where, with the help of Meyer Lansky, he set up business. In 1951 he opened Binion's Horseshoe. Never a Rat Pack hangout, the Horseshoe is the bastion of old-time serious gambling, the last family-owned casino in America and the gem in downtown Las Vegas's desperate, and probably doomed, attempt to compete with the Disneyfied palaces that line the Strip to the south. Benny Binion died in 1989, leaving his children to fight over his casino. For a time it was run by Ted Binion, one of his sons, who, like Benny, was known for never backing down from a bet. But Ted had problems, a fondness for heroin being only one of them. A historian of gambling tells McManus that ''Ted possessed all of his father Benny's bravado and none of his insight and good sense.'' This became apparent in 1998, when he was murdered in a particularly gruesome fashion by his girlfriend, a former topless dancer, and her other boyfriend, one of Ted's ostensible friends.

Their murder trial coincides with the 2000 World Series of Poker, and they are convicted just after the tournament ends. McManus integrates an account of the killing into his narrative of the poker competition. The latter is as tension-packed as any thriller. He antes up his $4,000 to buy into a qualifying game. Amazingly, he wins and collects a chit for his $10,000 entry fee. Then he goes off to Neiman Marcus to check out the story that Binion's girlfriend discussed Ted's impending death with her manicurist a week before his murder. When McManus finds the beauty salon closed, he buys his wife a $2,600 diamond ring even though he doesn't actually have any of that $10,000 he won. You can't fault that logic either, especially when he later confesses to the lap dance he bought, as part of his ''research'' at the aptly named club Cheetahs.

Then it's on to the World Series of Poker itself, the winner of which will take home more than $1 million. The Series is -- what else? -- a melting pot. ''Among toned jocks,'' McManus writes, ''we have about equal numbers of the obese and the skeletal, plus people in bare feet and wheelchairs and dance shoes. . . . C.E.O.'s and dot-com zillionaires versus call girls, masseuses and poker dealers. We also have gay men and lesbians, cowgirls and golfers and artists, black poker professionals and Jewish physicians, Jewish pros and black docs, at least one Aramaic scholar and rabbi, and several Vietnamese boat people.'' All have paid, or won, the $10,000 entry fee, and all know what they're doing.

And so the amateur sits down to play again, with his shades and baseball cap and, for luck, his ''brag book'' filled with photographs of that understanding wife and their two young daughters. And the album works. By the third day he has won $450,000. Anyone who has ever stood around stuffing $100 bills into a slot machine and paused to consider that back home he or she will drive across town to save a nickel on a gallon of gas will appreciate the surreal quality money can take on in Las Vegas. This happens to McManus, who, when contemplating that $450,000, realizes it is seven times his salary. ''In real life,'' he writes, ''I make sure the kitchen faucet handle points toward Cold before rinsing a sippy cup.''

At one point he wins $866,000 in a single hand and takes the lead. And now we're into this with him, willing to pore over his recounting of hand after hand and becoming intimate with the strategies of marathon no-limit poker. He's not afraid to admit his overwhelming fear when a hand is decided on the turn of a final card, ''fifth street.'' But for all his self-deprecation, McManus is smart. He knows that the professionals have him marked as a weak player, so he doesn't play like one. Or at least he tries hard not to. Then, abruptly, inevitably, the streak ends. Having made the final table on the fourth day, he finishes fifth (out of 512) and walks out with just under $250,000 -- and, even better for a writer, a great story.